Machine tools controlled by numerical control apparatus ordinarily are provided with apparatus for monitoring abnormal operations in the tool. The abnormal operation monitoring apparatus must, when monitoring, discriminate which tool is actually performing the machining. Information necessary for discrimination of tools is instructed into the numerical control (NC) tape in the form of tool change signals (M code), tool number signals (T code), and the like.
NC tapes record machining information in blocks comprising single performance instructions including position instructing signals (X code, Y code, Z code, and the like), a rotation speed instructing signal (S code), a tool change signal (M code), a tool number signal (T code), and the like.
The system used for supplying tool information from a numerical control apparatus to an abnormal operation monitoring apparatus according to the conventional technique is shown in FIG. 1. In the numerical control apparatus 1 of FIG. 1, the T code of one block of machining information is decoded and outputted as the BCD signal, for example, by operation of relay contacts. Supposing that the T code is expressed by, for example, the two digit number T02, only one signal T.sub.12 is put ON among the contact signals T.sub.28, T.sub.24, T.sub.22, T.sub.21, T.sub.18, T.sub.14, T.sub.12, and T.sub.11, and the other contact signals are put OFF. These contact signals are supplied to the abnormal operation monitoring apparatus 3. In the abnormal operation monitoring apparatus 3, a receiving circuit 31 receives the T code contact signals and supplies them to a T code processing circuit 32. The T code processing circuit 32 decodes the supplied T code signals, discriminates the selected tool, and supplies the tool discrimination signal to a selected tool processing circuit 33. The selected tool processing circuit 33 supplies the data necessary for detecting the abnormal operation in the tool to an abnormal operation detecting circuit 34. The abnormal operation detecting circuit 34 compares the operation data from the machine tool 2 with the above-mentioned data for detecting the abnormal operation in the tool so as to detect whether or not there is an abnormal operation in the tool.
However, conventional numerical control apparatus suffer from the problem that when remodeling an existing installation to provide it with a tool abnormal operation monitoring apparatus, it is necessary to design the circuit construction of the abnormal operation monitoring apparatus, the processing of the abnormal operation detecting data, and the like, individually for each installation. This design work is very troublesome.